The present invention relates in general to wireless communication systems, and in particular to beamforming techniques usable in multiple-input, multiple output (MIMO) wireless communication systems.
As used herein, the term “beamforming” refers to a variety of techniques for exploiting interference effects to improve reception of a signal. Traditionally, beamforming entails sending the same signal from multiple antennas with appropriate phase and/or amplitude adjustments such that a strong signal is detected at a receiver.
In MIMO (multiple input, multiple output) communication systems, a single data stream is divided into multiple spatial streams that are transmitted over multiple antennas in parallel. The receiver receives a linear combination of the spatial streams, which it decodes to reconstruct the original data stream. Parallel transmission of multiple spatial streams generally improves the data rate of the communication system as compared to more traditional SISO (single-input, single-output) systems. Further increases in the data rate can be attained by using orthogonal frequency division multiplexing (OFDM) or similar techniques within each spatial stream.
Beamforming may be employed in a MIMO system. With beamforming, instead of using each transmit antenna to transmit a spatial stream, the transmit antennas each transmit a linear combination of the spatial streams, with the combination being chosen so as to optimize the response at the receiver. More specifically, consider a MIMO system with a number (Nt) of transmit antennas and a number (Nr) of receive antennas that transmits a number (Nss) of spatial streams in parallel using a number Nc of OFDM subcarriers n. In the absence of beamforming, the Nr-component vector y(n) of signals received for subcarrier n may be expressed as:y(n)=H(n)x(n)+n(n),   (Eq. 1)where x(n) is an Nt-component vector of signals transmitted for subcarrier n, H(n) is an Nr×Nt matrix representing the effect of the communication channel for subcarrier n, and n(n) is an Nr-component vector representing the noise seen per receive antenna for subcarrier n.
A beamforming transmitter applies weights to signals x(n) prior to transmission, where each weight is a complex scalar. In general, assuming now that x(n) is an Nss-component vector, the beamforming weights for subcarrier n are represented as an Nt×Nss matrix W(n), and Eq. 1 becomes:y(n)=H(n)W(n)x(n)+n(n) (Eq. 2)
Various beamforming techniques for MIMO communication systems are known in the art. In general, such techniques can be classified in two categories. Techniques in one category attempt to equalize the channel at the transmitter, e.g., by selecting W(n) such that the product H(n)W(n) approximates an identity matrix, so that an AWGN-like performance is seen at the receiver. Minimum mean-square error (MMSE) techniques, e.g., as described in U.S. Pat. No. 6,144,711, are of this kind.
MIMO beamforming techniques in another category attempt to orthogonalize the channel at the transmitter side, e.g., by selecting W(n) such that the product H(n)W(n) is approximately a diagonal matrix, in order to improve receiver performance. Singular value decomposition (SVD) techniques are of this kind.
The extent to which beamforming improves receiver performance depends on how the weights W are selected. Improvements in beamforming techniques will result in improved performance and are therefore desirable.